In a "normally black" type twisted nematic liquid crystal display device, a 90.degree. twist of the major axes of the molecules is achieved by placing a liquid crystal layer between a first transparent electrode subjected to orientation treatment in a first direction and a second transparent electrode subjected to orientation treatment in a second direction, perpendicular to the first direction. A first transparent polarizing plate having a direction of polarization coinciding with the first direction is provided outside the first electrode and a second transparent polarizing plate also having a direction of polarization coinciding with the first direction is provided outside the second electrode. When a voltage is not applied between the first and second electrodes, light is not able to pass through the liquid crystal layer due to the 90.degree. twist of the major axes of the molecules of the liquid crystal layer. When an excitation voltage is applied between the first and second electrodes, the major axes of the molecules of the liquid crystal are all made parallel to an electric field created by the applied voltage. Thus, when a voltage is applied, incident light passes through the liquid crystal layer. In such a liquid crystal display device, the optical path length difference (.DELTA.nd) of the liquid crystal layer is set in such a manner that the transmission factor through the liquid crystal layer with no voltage applied to the electrodes is minimized. The optical path length difference may also be referred to as "retardation", that is, the product of the thickness (d) of the liquid crystal layer between the first and second electrodes (i.e. the thickness of the layer) and the double refraction index (.DELTA.n) of the liquid crystal layer. In a normally dark display, the display screen is observed from its front face and the second polarizing plate is viewed. The second polarizing plate is normally not transmitting any light.
Japanese Published Unexamined Patent Application No. 62-180327 discloses a liquid crystal display panel in which the difference in the optical path length of its liquid crystal layer is made to be in a range from 0.7 .mu.m to 0.9 .mu.m and the difference between the polarizing angle due to the polarizing plate and the rubbing angle is made to be in the range from 20.degree. to 50.degree..
Japanese Published Unexamined Patent Application No. 62-240928 discloses a liquid crystal optical shutter in which the product of the thickness of its liquid crystal layer and the double refraction index of the liquid crystal layer is between 0.18 .mu.m and 0.4 .mu.m.
Until now, in a arrangement for setting the optical path difference (.DELTA.nd) of the liquid crystal layer which is used in the above-mentioned "normally black" type twisted nematic liquid crystal display device, the contrast ratio is a maximum when the display screen is observed from the front face. However, the black level (color of background) and the contrast ratio vary with the change in the visual angle. In these displays, the liquid crystal layer must be thin. Accordingly, if small errors exist in the thickness of the layer, the contrast and the chromaticity will vary.
The technology disclosed in Japanese Published Unexamined Patent Application No. 62-180327 (see above) is applicable only to the so-called simple matrix-type liquid crystal display devices. The technology relates to a TN type liquid crystal display device using a bias rubbing method in which the twist of the axes of the molecules of its liquid crystal layer is not necessarily 90.degree., and the technology is not applicable to a "normally black" type twisted nematic liquid crystal display device. In a "normally black" twisted nematic liquid crystal display device, if the optical path difference (.DELTA.nd) is between 0.7 .mu.m to 0.9 .mu.m, the leakage of light during interruption of the electric field is large, the absolute value of the contrast ratio is low, and the color of the background varies depending on the visual angle. Thus, a display of this type is not applicable to normally black displays.
The technology described in Japanese Published Unexamined Patent Application No. 62-240928 (see above) is used to enhance the optical response speed of a liquid crystal shutter, and it is not applicable to a liquid crystal display device having a large screen. In order for the optical path difference (.DELTA.nd) to be 0.18 .mu.m and 0.4 .mu.m, it is necessary for the thickness of the liquid crystal layer, that is, the thickness (d) of the cell to be between 1.8 .mu.m to 4 .mu.m if the double refraction index (.DELTA.n) of the liquid crystal layer is 0.1. It is technically difficult to realize this thickness ranging over a large area, for example, in a range where errors must be within .+-.0.2 .mu.m.
The present invention is intended to provide a "normally black" type twisted nematic liquid crystal display device in which, when the visual angle is varied, the variation in its contrast and black level (color of background) is relatively small for very small variations in the thickness of the liquid crystal layer, and, moreover, a display in which a sufficient contrast ratio can be obtained.